Polydipsic hyponatremic schizophrenics & 3 control groups will receive an infusion of the psychostimulant methylphenidate to assess if previously defined alterations in vasopressin secretion are linked to psychosis. We predict the relationship between change in psychotic symptoms & vasopressin secretion will be shifted to the left in the polydipsic hyponatremic schizophrenics relative to the other groups.